1. Field of the Invention
The present invention relates to encoders, and more specifically, to an encoder that optically detects positional information of a scale for measurement with respect to a beam-splitting optical system.
2. Description of the Background Art
Conventionally, a method, in which by irradiating a light beam from a detection head to an optical scale that has a periodic structure like a phase diffraction grating and receiving the light beam with a light-receiving element, a relative positional relation between the detection head and the optical scale and a relative motion direction of both of them are measured, is used in general.
In the method that has been most commonly used recently, periodic signals whose phases are different by 90-degrees from each other are detected from a light beam via the optical scale, and a relative positional relation between the detection head and the optical scale and a relative motion direction of both of them are measured based on a relation between amplitude and phase of the two signals. Further, in order to solve reduction in detection resolution occurred in this method that is caused by reduction in the S/N ratio of the detection signals due to noise, the method described in U.S. Pat. No. 6,639,686 can be used. According to the method described in U.S. Pat. No. 6,639,686, the detection signal is modulated by oscillation of a light beam, and when measuring the relative positional relation between the detection head and the optical scale and the relative motion direction of both of them, the signal is demodulated. The S/N ratio can be improved by such a demodulating process, and as a consequence, the detection resolution can be improved.
In the method described in U.S. Pat. No. 6,639,686, however, in order to oscillate the light beam, an optical element that can be oscillated is placed in an optical path, and the optical element is oscillated. In this case, when drift occurs in the oscillation center of the light beam, the detection result includes error, which could hinder the high-precision position detection.
Further, as a conventional common optical encoder, an encoder by a diffraction interference method is known: which is equipped with a diffraction grating that moves along a movable body and has gratings formed at regular intervals orthogonally to a moving direction, an irradiation optical system that irradiates a coherent beam to the diffraction grating, and a detector that makes the diffraction lights diffracted by the diffraction grating interfere and detects the intensity change of the interference light; and detects a moving distance of the diffraction grating (e.g. refer to Kokai (Japanese Patent Unexamined Application) No. 2005-003438 and the like).
In such an encoder, there could be the case where a member that holds the light source or the diffraction grating expands/contracts due to heat generated from the light source and the relative positional relation between the light source and the diffraction grating changes over time. This positional deviation causes detection error of the moving distance of the diffraction grating.